Dip and bake insulation coatings for use in high performance motors and generators were taught by Sprengling et al., in U.S. Pat. No. 3,405,091. There, a mixture containing monomeric chloromethyl diphenyl oxide, unsubstituted diphenyl oxide, and from about 0.008% to 20% of catalyst selected from one of Friedel-Crafts catalysts, such as AlCl.sub.3, ZnCl.sub.2 or BF.sub.3 ; silica, such as 5 micron Minusil SiO.sub.2 powder; diatomaceous earth; bentonite; organic soluble metallic chelates, such as ferric acetyl acetonate; or p-toluene sulfonic acid, was heated, at from about 50.degree. C. to 150.degree. C., until all the chlorine reacted, followed by vacuum stripping of unreacted diphenyl oxide, to provide a soluble, fusible novolac resin product. Where SiO.sub.2 powder or metallic chelates were used as catalysts, HCl could be added as an accelerator capable of adsorption on the catalyst surface, releasing iron impurities which improved overall catalytic effect.
The novolac resin of Sprengling et al. was then reacted with methoxymethyl diphenyl oxide monomer, optionally using the same amounts and kinds of previously used catalysts, only to a point where the product was still soluble in common solvents, and would still contain unreacted methoxymethyl functional groups, which upon further heating, at about 200.degree. C. for about 4 hours, would react to form a cured thermoset insulation, with thermal breakdown times greater than 200 hours at 300.degree. C. The use of methoxymethyl or other alkyl substituted diphenyl ethers for the chloromethyl diphenyl oxide, was taught as making it impractical to strip off free diphenyl oxide, which would contribute to weight loss during long, high temperature thermal aging, because it would evaporate at high temperatures. While this provided excellent insulation products, the formation of chloromethyl ethers in the process caused health concerns.
Plepys et al., in U.S. Pat. No. 4,154,767, and Nelson, in U.S. Pat. No. 4,188,473 taught poly(methylene diphenyl ether) laminating, molding, or film-forming materials, which could be cured or thermoset by heat, and which were produced without the use of chloromethyl diphenyl oxide. In those patents, diphenyl oxide was mixed with formaldehyde, water, methanol, and a strong acid catalyst, selected from one of sulfuric acid, phosphoric acid, p-toluene sulfonic acid, perchloric acid, diphenyl oxide sulfonic acid, or strong acid cation exchange resin, and the mixture was heated at from about 50.degree. C. to 250.degree. C., followed by vacuum stripping of unreacted diphenyl oxide, to provide a mixture of ortho and para methoxymethyl diphenyl oxides, para-bis(phenoxymethyl)methane, and a 40% mixture of other diphenyl oxides. This reaction product could be polymerized by further reaction, for 1 hour to 2 hours, at from about 140.degree. C. to 165.degree. C., with from about 0.2 wt.% to 0.3 wt.% of a Friedel-Crafts acid catalyst, particularly an alkylated diphenyl oxide disulfonic acid, to form a cured thermoset resin. These resins, however, have been found to have a relatively short shelf life, tending to form a flaky precipitate after about 3 months, which is almost impossible to redissolve. What is needed is a non-toxic, high bond strength insulation, having good room temperature storage stability.